This invention relates in general to coupling assemblies for use in drilling directional well boreholes. More specifically, the invention relates to a coupling assembly such as a bent sub for connecting two cylindrical members at an angle whereby the members can be manipulated from the surface to cause the borehole to be drilled in a selected direction.
For various reasons it is desirable to change the direction of the drilling of the borehole of a subterranean well. For example, the change in direction may be required to straighten a well due to the deflection of the drill bit from the desired direction due to particular rock strata. In other instances, the change in direction is intentional in order to reach a formation that is laterally displaced from the existing location of the borehole.
Directional drilling is a procedure employed when it is necessary to change the direction of a borehole. One of the most common methods for changing direction has been the insertion of a bent sub in the drilling string at a point above a downhole motor which drives the rotary drill bit. The downhole drilling motor uses the energy of drilling fluid flowing through the drill string to rotate the bit. Such motors may be turbine motors or progressive cavity motors. The latter type of motor usually has a rotor connected to a bearing shaft by a connecting rod. The motor, the connecting rod, and the beating shaft are housed within a motor housing, a connector housing, and a bearing housing, respectively.
The bent sub is rigidly connected at one end to the drilling string and has its other end angularly disposed relative to the axis of the drilling string so that when the motor and supported drill bit are rigidly supported by the other end of the bent sub, the rotational axis of the drill bit is angularly inclined relative to the axis of the drilling string. The slight angle deviation from the longitudinal axis of the drilling string caused by the bent sub is often referred to as the off set angle. The bent sub may also be used to connect the motor housing to the bearing housing.
In practice, a vertical borehole is drilled to a predetermined depth and then the drill string is withdrawn to insert a bent sub having the desired off set angle between the end of the drill string and the downhole motor. The drill string is then tripped back into the borehole such that the longitudinal axis of the drill bit is now at an angle to the original borehole thereby altering the direction of the borehole. Typically, each bent sub has an off set angle ranging from about one-quarter of one degree in one-quarter degree increments to about two degrees.
A typical bent sub consists of a tool joint with a tubular section having threaded ends. The axis of the threaded ends is coincident with the axis of the tubular section. The axis of the opposite end is at an angle to the axis of the tubular section. When the bent sub is used to connect pipe sections, the axis of adjacent drill pipe sections will have the angle imposed with respect to each other established by the bent sub. The bent sub, when used with a downhole motor, usually is placed between the drill collars which are used to impart the desired weight on the bit and the downhole motor.
To effect a desired change in drilling angle, which generally is on the order of a fraction of a degree, it is necessary to remove the motor and drill bit from the end of the drill string and either replace the bent sub with another bent sub having the desired angular deviation or to use an adjustable bent sub or housing. U.S. Pat. No. 4,522,272 discloses a bent housing for incorporation in a drilling string which is adjustable to provide a range of angular positions of the rotary drill bit relative to the axis of the drilling string.
U.S. Pat. No. 4,077,657 is directed to an adjustable bent sub for selectively setting the axial alignment of pipe sections from axial coincidence to an obtuse angle between the pipe sections. The bent sub is in two separate cylindrical parts. The adjacent ends of the parts are formed so that the facing or mating ends are each at a mating plane which is at the same acute angle to the axis of the pans where they are coincident. In one position, the axis of the parts are coincident with each other. By rotating one of the pans about the axis of the other pan, the axis will become inclined relative to each other. Each of the mating pans has teeth that engage one another and the pans are marked off so that the degree of inclination may be adjusted per the indicator marks by rotatively moving one mating part relative to the other. A threaded internal mandrel locks one part of the bent sub to the other pan.
U.S. Pat. No. 4,745,982 is also directed to a bent sub used in directional drilling. The bent sub includes an elongated main mandrel which has a fixed bent angle from the longitudinal axis thereof. Mounted to the exterior of the mandrel is a sleeve in a bent sub housing. The ends of the sleeves and the housing face one another and have a dog clutch arrangement with teeth for inter-engagement of the sleeve and the housing. Any given position between the sleeve and the housing with the teeth engaged is maintained in that position by retaining threaded sleeves which axially lock the sleeve and the housing together.
U.S. Pat. No. 4,813,497 utilizes the bent sub of U.S. Pat. No. 4,745,982 with the addition of indicating means between the adjustable pans of the bent sub to indicate the rotational position of a second member with respect to a first member of the bent sub.
U.S. Pat. No. 4,817,740 is directed to a tool which affects change in the direction of a downhole rotary drilling tool and includes a bent housing disposed between the bottom end of a downhole motor and a tubular housing connected to the drilling tool. The bent housing is angularly adjustable relative to the motor housing and the connecting housing is provided with an eccentric external surface upon which an annular eccentric stabilizer is adjustably mounted. The degree of angulation between the motor and the stabilizer is controlled by a series of annular split rings with different thicknesses. The split ring is disposed between the motor and the stabilizer. If a change of inclination is required between the motor and the stabilizer, a thicker or thinner ring is placed between the motor and the eccentric member. After the ring change, the three sections are tightened together against the ring.
Other U.S. patents describing connections with downhole motors include U.S. Pat. Nos. 5,168,943; 5,048,621; 5,052,501; 5,029,654; 4,067,404; and 4,641,717.
Prior art bent subs lack the ability to take high torsional and bending impact loads without the connections backing off or fatigue cracking. Prior art bent subs also do not provide positive stops. The present invention overcomes the deficiencies of the prior art.